INDUSTRIAL AND COMMERCIAL HEATING PIPEWORK
CORROSION GUIDANCE IN THE PIPELINE
Failure of pipework in heating and cooling systems can result in expensive remedial work. New guides and standards should help to reduce the risk of corrosion in closed water systems, says Reginald Brown
If a pipe fails due to corrosion, then the cost of damage to the building and remedial work to preclude further failures can be enormous
F
or building services engineers, water is both a required service for an occupied building and an effi cient and virtually free medium for heat transfer.
Unfortunately, mains water, which contains dissolved oxygen, is also potentially corrosive to metals in the pipework system and the plant it passes through. If a pipe fails due to corrosion, then the cost of damage to the building and remedial work to preclude further failures can be enormous. In principle, it should be possible to select a set of non-corrodible materials and use untreated water as the heat transfer medium, without any additional measures to avoid corrosion. In small and simple systems this may be a feasible approach, though we should also consider other possible issues, such as biofouling. In larger and more complex systems, construction using wholly non-corrodible or corrosion resistant materials may be impractical or simply too expensive. Generally, there will be some corrodible material within the system and it will be necessary to manage the corrosion risk for that material. There are several implications of using corrodible materials within a closed heating or cooling system:
The corrosion process will begin as soon as the material is wetted, usually at the pipework pressure testing stage
The period between initial wetting and commissioning should be kept as short as possible as the system cannot be completely protected in the absence of fl ow
The pre-commission cleaning phase may need to include chemical cleaning to remove oxidation debris and biofi lm for subsequent water treatment to be effective
Failure to monitor and maintain water treatment during commissioning, and
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remedial works when oxygen levels are likely to be high, may have dire consequences
Oxygen levels in use must be reduced as quickly as possible and kept as low as possible by suffi cient system pressurisation, effective venting and minimising the addition of fresh water
All areas of the system must frequently receive fl ow to allow the water treatment chemicals to reach the vulnerable surfaces
The water treatment regime must be maintained through the life of the building
The factors that lead to corrosion and potential failure of pipework systems have not changed in 100 years or so, since piped heating systems became common, so why do corrosion failures seem to be increasing, sometimes even before the building is handed over? The major factors are changes in the materials and system design, coupled with a lack of awareness of the risks. Traditional steel pipework systems are
readily corroded by fresh water but can sustain signifi cant surface corrosion without danger of perforation or detriment to the overall life of the system. Also, the corrodible surface area is large, relative to the volume of water in the system so that, in closed systems the oxygen level will drop quickly as it is used up in the chemical corrosion reaction, and the corrosion is, to some extent, self-limiting. This is why we often see old steel systems that have never been chemically treated but are in remarkably good condition. Of course, this can only happen when there is no fresh water or air being added to the system. If that is not the case, then chemical inhibitors are necessary to reduce the rate of corrosion in the continuing presence of dissolved oxygen.
Rapid pitting corrosion This is particularly important when using thin wall carbon steel pipe, which is attractive to contractors on the grounds of cost and speed of installation, but rather less tolerant of corrosion than traditional steel pipe. It is particularly susceptible to rapid pitting corrosion if oxygen continues to enter the system, and can potentially fail within months if maltreated. One manufacturer suggests that the corrosion risk increases when the oxygen concentration exceeds 0.1 mg/l (presumably in the absence of an inhibitor). The concentration of oxygen in tap water
is around 10 mg/l, so particular attention must be paid to the avoidance of fresh water additions (or air ingress through other routes), plus a well-monitored and effective chemical inhibitor regime.
February 2013 CIBSE Journal 53
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